1. Field of the Invention
This invention relates generally to electrically and mechanically interconnecting one electronic component to another electronic component to form an electronic assembly or module and, in particular, to stretching or otherwise elongating the solder joints of an existing module or a module being fabricated to form elongated solder joints having enhanced mechanical and electrical integrity and reliability.
2. Description of Related Art
Forming an electronic package assembly whereby an electrical component such an integrated circuit chip is electrically connected to a substrate such as a card, or board, another chip or another electronic part is well-known in the art. Surface mount technology (SMT) has gained acceptance as the preferred means of joining electronic package assemblies. The following description for convenience will be directed to joining ceramic electronic components such as multilayer ceramic components as exemplified by integrated circuit chips to printed circuit cards or boards.
Multilayer ceramic electronic components are typically joined together by soldering pads on a surface of one of the electronic components to corresponding pads on the surface of the other component. Control Collapse Chip Connection is an interconnect technology developed by IBM as an alternative to wire bonding. This technology is generally known as C4 technology or flip chip packaging. Broadly stated, one or more integrated circuit chips are mounted above a single or multiple layer ceramic substrate or board and pads on the chip are electrically or mechanically connected to corresponding pads on the other substrate by a plurality of electrical connections such as solder bumps. The integrated circuit chips may be assembled in an array such a 10.times.10 array.
In the C4 interconnect technology, a relatively small solder bump is attached to pads on one of the components being joined. The electrical and mechanical interconnects are then formed by positioning the corresponding pads on the other electronic component adjacent the solder bumps and reflowing the bumps at an elevated temperature. The C4 joining process is self-aligning in that the wetting action of the solder will align the chip bump pattern to the corresponding substrate pads.
A myriad of solder structures have been proposed for the surface mounting of one electronic structure to another. Typical surface mount processes form the solder structures by screening solder paste on conductive, generally metallic pads exposed on the surface of the first electronic structure or substrate. A stencil printing operation is used to align the contact mask to the pads. The solder paste is reflowed in a hydrogen atmosphere and homogenizes the pad and brings the solder into a spherical shape. The solder spheres on the substrate are then aligned to corresponding pads on the electronic structure or board to be connected thereto. After alignment, the substrate and board go through a reflow operation to melt the solder and create a solder bond between the corresponding pads on the substrate and other electronic component. The interconnection is typically in a form of a double truncated sphere as shown in FIG. 6.
Other known surface mount technology uses solder balls rather than solder paste to provide the solder connecting structures. By using solder balls, a more exact and somewhat greater quantity of solder can be applied than from screening. The solder balls are aligned and are held to a substrate and melted to form a solder joint on a conductive pad of the substrate. As before, the substrate with the newly joined solder balls is aligned to the board to be connected therewith and the solder balls are then reflowed to form a solder bond between the two substrates. The use of a copper ball surrounded by eutectic solder is also used as a solder joint structure for attaching a multilayer ceramic (MLC) substrate to a PC laminate wherein the ball serves as a standoff.
C4 joint technology, commonly referred to as ceramic ball grid array technology (CBGA), offers advantages of low cost, and a low inductance surface mounting interconnection. However, a thermal expansion mismatch between the chip and substrate will cause a shear displacement to be applied on each solder connection. Over the lifetime of a module, this leads to an accumulated plastic deformation and decreases the lifetime of the module. The typical solder joint having the shape of a doubly truncated sphere which is truncated at each pad is known to have a limited lifetime due to the thermal mismatch between the substrates and the shape of the bump. It is known that changes in shape away from a spherical segment can produce enhanced mechanical properties and testing has shown an order of magnitude difference in fatigue life between an hour-glass shape and a barrel-shape (double truncated spherical) joint. The fracture location of the hour-glass joint shifted to the center of the joint instead of at the pad-joint interface.
Stretched solder joints have been fabricated by a number of techniques including using different solders on the same chip. Solder columns have also been stacked to achieve improved fatigue life. Two solder alloys have been used in combination wherein the surface tension of a larger bump is used to raise and stretch the smaller area of the joints. Another approach involves stacking C4 bumps using polyimide interposer, but this technique adds considerable cost and manufacturing complexity without improved benefit.
A centrifugal force technique has also been employed wherein modules having C4 solder joints are heated and spun such that the C4s are elongated. This technique has obvious manufacturing difficulties when using large printed circuit cards. Brackets attached to one of the substrates has also been used to stretch the C4 joints as shown in U.S. Pat. No. 5,148,968. Basically, the bracket, when activated by heat or other means, raises one of the substrates in the z-axis stretching the C4 connection. Some of the brackets, however, becomes an integral part of the final structure requiring the packaging design to incorporate the bracket and related fixturing.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of forming an elongated solder joint between two solder interconnected substrates in an electronic module to enhance the mechanical and electrical integrity and reliability of the module.
It is another object of the present invention to provide a chip or bracket stretching device for stretching solder joints to form an elongated solder joint between two solder interconnected substrates of an electronic module to enhance the mechanical and electrical integrity and reliability of the module.
A further object of the invention is to provide electronic modules having solder joints having enhanced mechanical and electrical integrity and reliability.
It is yet another object of the present invention to provide an electronic component assembly or module made using the method and stretching device of the invention.
Other objects and advantages of the invention will be readily apparent from the description.